1. Technical Field
The present invention relates generally to the power train of a motor vehicle. More particularly, the present invention relates to a power transfer unit for distributing power to the rear wheels of the vehicle.
2. Background Information
In the past, most automobiles in the United States utilized a rear wheel drive power delivery scheme. In adapting these rear wheel drive schemes into four wheel drive applications, a transfer case was, and often still is, positioned at the output of the transmission assembly. When engaged, the transfer case diverts a portion of the power coming from the transmission assembly from the rear wheels to the front wheels.
Currently in the United States, a significant portion (if not a majority) of new automobiles are front wheel drive based vehicles. In a front wheel drive vehicle, both the engine and the transmission assembly are typically transversely oriented in the vehicle. By positioning the power plant and transmission assembly transversely in the vehicle, more direct coupling of the transmission assembly to the vehicle's transaxle and front wheels can be achieved.
With front wheel drive vehicles themselves becoming a mature market, a recent trend in the automobile industry has been to adapt these front wheel drive schemes for all or four wheel drive applications. This is accomplished by providing a power transfer unit that diverts a portion of the power from the front wheels to a rear wheel drive shaft and subsequently to the rear wheels.
Several criteria affect the design of power transfer units. Typically, the input portion of the power transfer unit is coupled to the output of the transmission, and the output portion of the power transfer unit is coupled to the rear wheel drive shaft. In achieving this, the power transfer unit must accommodate a certain amount of lateral offset, the lateral distance between the axis of the rear propeller shaft and the mounting face of the transmission assembly. As such, the primary components of the power transfer unit, namely, the inner gear sets, are located some distance from the transmission, which requires using an extended housing to couple the power transfer unit to the transmission.
In addition, contaminants remaining in the power transfer unit from the production of the power transfer unit and additional contaminants generated by the wear of the internal components during the operation of the unit may reduce the life of the gears and support bearings. Moreover, the gear sets, associated bearings, and the proximity of the engine, transmission and exhaust components may generate more heat than can be rejected from the main cavity or housing of the power transfer unit. In these units, additional cooling is typically required through the use of cooling fins, auxiliary liquid-air coolers, or auxiliary liquid-liquid coolers. Some units even employ a pumped heat exchanger which transfers heat from the power transfer unit to the engine coolant.
To prevent air pressure or vacuum from building within the power transfer unit, the power transfer unit must allow venting from the interior of the unit to the external atmosphere while at the same time preventing lubricant from escaping the enclosure. During the operation of some units, lubricant is occasionally atomized into droplets or churned into foam. In these units, some mechanism must be employed to separate the lubricant from the ventilated air. For instance, certain power transfer units use a separation cavity or labyrinth that allows lubricant droplets or foam to settle out of the air before the air is ventilated to the atmosphere. These cavities may be cast into the housing or formed adjacent to the vent by attaching a separate shield.
From the above, it is seen that there exists a need for an improved power transfer unit with enhanced heat rejection, contamination collection, and ventilation.